One of the well reported characteristics of the muscles of older people is an atrophy of type II muscle fibres1. This selective atrophy results in a change in the contractile protein profile as a whole towards that of a slower muscle2, which coupled to a loss of muscle mass and force generating capacity contributes to the decline in power output seen in older people. However, the contribution of the inherent ageing process to this phenomenon is unclear as the majority of studies have evaluated sedentary people, where the deleterious effects of inactivity are superimposed on the inherent ageing process. To overcome this limitation the use of highly active older individuals has been advocated as model to study the inherent effects of ageing3. The aim of the present study was to investigate whether there was an effect ageing on type II muscle fibre size in cyclists aged 55 – 79 (60 males, 30 females) years who maintain high levels of physical activity. Explosive power output was also determined. Following local anaesthesia (2% lidocaine) muscle biopsy samples were obtained from the vastus lateralis muscle using the Bergström needle technique, with applied suction. Samples were frozen in liquid nitrogen-cooled isopentane and serial sections obtained. Samples were stained for myofirillar ATPase activity after pre-incubations at pH 4.3, 4.6 and 10.2. Muscle fibre distribution and CSA were determined. Peak power output was obtained from a maximal sprint test on an inertial ergometer2 and normalized to lean body mass as determined from dual X-ray absorptiometry (DXA) scanning.Fibre distribution was similar between males and females with an average of 31.7 ± 13.1% type II fibres identified. Whilst females (Type I = 4709±1443 μm2; Type II 3344±1480 μm2) had smaller fibres than males (Type I = 6451±1354 μm2; Type II 5624±1984 μm2; P<0.001 in both cases), neither the cross-sectional area of type I nor II fibres changed as a function of age (P>0.05; Fig 1). The males generated on average 1058±143 Watts of power which was significantly greater than the females (796±109 W; p<0.001). When normalised to lean body mass, there was no difference in peak power output between males and females (54.4±6.6 W/kg) and as a group, there was modest decline in peak power output with age (P = 0.025, r = .237, n=90; Fig. 2).The results of this study in highly active individuals, show no decline in type II fibre cross-sectional area, at least in the age range studied and with only modest declines in power output. The data support the hypothesis that type II fibre atrophy, reported in previous studies, relates to inactivity superimposed on the ageing process rather due to an inherent effect of ageing per se.